Method of producing forgings having excellent tensile strength and elongation from steel wire rods

ABSTRACT

A method of producing forgings includes the steps of: forming forgings from a steel wire rod; heating the forgings to a temperature range of 830˜900° C.; subjecting the forgings to first tempering at a temperature range of 100˜300° C. after the heating at 830˜900° C.; and subjecting the forgings to second tempering at a temperature range of 300˜400° C, after the first tempering. In the first tempering, the forgings are quenched in a salt bath having a temperature in the range of 100˜300° C., and are tempered for 60˜130 minutes. In the second tempering, the forgings are quenched in a salt bath having a temperature in the range of 300—400° C., and are tempered for 30˜150 minutes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of producing forgings from steel wire rods, more particularly to a method of producing forgings from steel wire rods, in which the forgings have a tensile strength of up to 160 kgf/mm² and an elongation of up to 16-21%.

2. Description of the Related Art

CNS3934 of the Chinese National Standards (CNS) specifies the standard properties that forgings should have. The highest grade of tensile strength for strong screws is 140 kgf/mm². However, due to the limitations encountered when using old materials and old manufacturing methods, it is difficult to produce such screws. Hence, many manufacturers do not produce screws of the highest grade of tensile strength.

In light of the aforementioned standard for strong screws, a method of producing screws that have the highest tensile strength has been developed by the applicant, and is disclosed in Taiwanese Patent No. I254656, titled “Method of Producing Forgings that have a Tensile Strength of up to 140 kgf/mm².” As shown in FIGS. 1 and 2, the steps involved in the method of producing forgings, as disclosed in Taiwanese Patent No. I254656, are summarized as follows:

-   -   (a) subjecting an alloy steel wire rod 1 to a spheroidized         annealing process;     -   (b) picking and coating the wire rod 1;     -   (c) subjecting the wire rod 1 to intermediate drawing;     -   (d) subjecting the wire rod 1 to a softening annealing process;     -   (e) picking and coating the wire rod 1 a second time;     -   (f) subjecting the wire rod 1 to skin-pass drawing;     -   (g) stamp-forging and roll-threading the wire rod 1 to form         forgings 2 (only one is shown in FIG. 1), such as screws;     -   (h) washing the forgings 2;     -   (i) heating the forgings 2 to a temperature range of 830˜900° C.         for 50˜90 minutes; and     -   (j) austempering the forgings 2 at a temperature range of         300˜400° C. for 30˜60 minutes.

After undergoing the heating process in step (i), the structure of the forgings 2 is changed from a pearlite structure to an austenite structure, and after undergoing the austempering process, the structure of the forgings 2 is changed from the austenitic structure to an acicular structure of lower bainite which has favorable mechanical properties, that is, good strength and toughness.

The forgings 2 produced by the aforementioned manufacturing method have a tensile strength of up to 140 kgf/mm², and a percent elongation of up to 9%˜14%, which meet the CNS3934 standard specifications for strong screws. However, since the forgings 2 have different dimensions, e.g., some have a diameter of over 24 mm, during the heating process, the large size forgings 2 easily produce a “mass effect,” i.e., a phenomenon in which the hardening effects of inner and outer parts of the steel differ during quenching. Thus, the larger the dimensions, the more difficult it is for the heat to spread uniformly from the inner to the outer parts of the steel. As such, during the austempering process, although the larger forgings 2 have outer portions that are easily changed into the lower bainite structure, the central portions thereof still have the structures of the ferrite plus medium pearlite. Hence, it is difficult for the forgings 2 with large dimensions to achieve the tensile strength of 140 kg/mm², so that the resulting products have insufficient hardness and mechanical properties.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a method of producing forgings that have a tensile strength of up to 160 kgf/mm²and an elongation of up to 16˜21%, and that also have a high degree of toughness and hardness.

According to the present invention, a method of producing forgings having a high tensile strength and a good elongation comprises the steps of: (a) forming forgings from a steel wire rod; (b) heating the forgings to a temperature range of 830˜900° C.; (c) subjecting the forgings to first tempering at a temperature range of 100˜30° C. after the heating at 830˜900° C.; and (d) subjecting the forgings to second tempering at a temperature range of 300˜400° C. after the first tempering.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment with reference to the accompanying drawings, of which:

FIG. 1 is a flow chart illustrating the steps involved in a method of producing forgings disclosed in Taiwanese Patent No. I254656;

FIG. 2 is a diagram illustrating changes in temperature with the passage of time during the manufacturing process of FIG. 1;

FIG. 3 is a flow chart illustrating the steps involved in a method of producing forgings according to the preferred embodiment of the present invention; and

FIG. 4 is a diagram illustrating changes in temperature with the passage of time during the manufacturing process of the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 illustrates the steps involved in a method of producing forgings 4 that have a tensile strength of up to 160 kgf/mm² and an elongation of up to 16˜21% according to the preferred embodiment of the present invention. These steps will be described in greater detail below.

In step (a), an alloy steel wire rod 3 is subjected to a first spheroidized annealing process at a temperature range of 680˜700° C., which is lower than the phase transformation temperature of the alloy steel wire rod 3. The wire rod 3 is made of a hardened wire material selected from the group consisting of nickel-chromium-molybdenum steel, such as SNCM439, and chromium-molybdenum steel, such as SCM445H (SAE4145H), SCM445, SCM440, or SCM440H. In this embodiment, the wire rod 3 is made of SNCM439 (JIS G4103; nickel-chromium-molybdenum steel corresponding to AISI 4340, SAE4340) with a chemical composition of 0.36˜0.43% C; 0.15˜0.35% Si; 0.60˜0.90% Mn; 1.60˜2.00% Ni; 0.60˜1.00% Cr; and 0.15˜0.30% Mo. This material conforms to the material testing regulation of the CNS3935 standard, and can thus achieve the required hardness.

In step (b), the wire rod 3 undergoes the process of picking and coating in a conventional manner.

In step (c), the wire rod 3 is subjected to intermediate drawing at a drawing ratio in the range of 20˜30%.

In step (d), the wire rod 3 is subjected to a second spheroidized annealing process at a temperature range of 600˜650° C.

In step (e), the wire rod 3 undergoes the process of picking and coating a second time after step (d).

In step (f), the wire rod 3 is subjected to skin-pass drawing at a drawing ratio of less than 5%.

In the present invention, the wire rod 3 undergoes the spheroidized annealing process twice and the drawing process also twice, so that the carbide substance of the steel becomes round and small to increase its softness and elongation, thereby facilitating a subsequent stamp-forging process. Because of the different conditions of the material of the wire rod 3 in terms of composition, thickness, etc., during the annealing and drawing processes, the process steps or the temperature may be increased or decreased as needed.

The purpose of the aforementioned spheroidized annealing process is to spheroidize the layered or netted structure of the carbide substance in the steel so as to improve the mechanical properties of the steel. The purpose of the drawing process, on the other hand, is to reduce the diameter of the wire rod 3. During the drawing process, the layered pearlite structure and the ferrite structure parallel to the cementite are displaced and deformed, and micro cracks occur along split surfaces of the cementite body, thereby resulting in spheroidization.

In step (g), the wire rod 3, after being subjected to the skin-pass drawing, is sent into a forging machine, and is stamp-forged to form forgings having preset outer contours. The forgings are then sent into a thread rolling machine to thread the surface thereof to form threaded forgings 4 (only one is shown in FIG. 3, hereafter referred to simply as “forgings”), such as screws.

In step (h), the forgings 4 are washed by initially using hot water to remove oily stains, then by using cold water to perform a final rinse of the forgings 4.

Prior to proceeding with step (i), the forgings 4 are pre-heated in a furnace at a temperature range of 550˜650° C. for 30˜90 minutes. In this embodiment, the pre-heating process is carried out in a furnace that is gradually heated to about 600° C. for 60 minutes. The pre-heating process may or may not be performed depending on the equipment used. For example, if a common fixed temperature furnace is used as heating equipment, the pre-heating process should be carried out. On the other hand, if a continuous type, temperature-adjustable heating equipment is used, the pre-heating process can be dispensed herewith.

In step (i), the forgings 4 are heated in a furnace at a temperature range of 830˜900° C. for 30˜120 minutes. The temperature in the furnace can be gradually increased, decreased, or kept constant. For example, the furnace may be heated gradually to six different temperature levels, such as 860° C., 880° C., 880° C., 880° C., 880° C., and 870° C. The heating lasts for 35 minutes so as to heat the forgings 4 to an austenite-stabilizing temperature until the forgings 4 are transformed completely to the austenitic structure. This is referred to as an austenitizing treatment.

In step (j), the heated forgings 4 are subjected to a first tempering process so as to reduce the temperature of the forgings 4 to a temperature range of 100˜300° C. for 60˜130 minutes. In this step, the heated forgings 4 are quenched in a salt bath which has a temperature in the range of 100˜300° C. The temperature of the salt bath is maintained substantially constant for 120 minutes. As such, the structure of the forgings 4 is transformed from the austenitic structure to a body-centered tetragonal (BCT) structure of martensite plus the structure of lower bainite. Prior to completion of the transformation into martensite, the next process step (step k) is carried out. An isothermal temperature salt (marquench, MQ) that belongs to the neutral salt bath is used in this embodiment.

In step (k), the forgings 4 are subjected to a second tempering process at a temperature range of 300˜400° C. for 30˜150 minutes. The forgings 4 are quenched in a salt bath which has a temperature in the range of 300˜400° C. and which is maintained substantially constant for 150 minutes, so as to transform the structure of the forgings 4. Thereafter, the forgings 4 are cooled to room temperature. During the salt bath process, the structure of the forgings 4 is transformed into an acicular structure of lower bainite, which is a non-layered structure of ferrite and fine cementite, and portions proximate to the central portion of the forgings 4 and the initially transformed martensite are subjected to the tempering effect. The relationship between time and temperature involved in the processing steps of the forgings 4 is shown in FIG. 4.

The quenching stress is thus eliminated, and the tempered martensite, which is a mixture of ferrite and fine cementite, is obtained. Hence, after the second tempering process, the structure of the forgings 4 becomes a mixed structure of lower bainite and tempered martensite. The mixed structure has the mechanical properties of high strength and good toughness.

Tests were carried out using SNCM439 Ni—Cr—Mo steel hardened wire material to produce the forgings 4 having M36 specification. A few samples of the forgings 4 were subjected to the aforementioned heating process steps. Through actual tests and measurements, the hardness of the forgings 4 was found to be 50˜51HRC, the elongation 16˜18%, and the tensile strength 160˜170 kgf/mm². Hence, it was confirmed that the forgings 4 have good mechanical properties.

From the aforementioned description, the advantages of the present invention may be summarized as follows:

Through the first tempering process, the structure of the forgings 4 is directly transformed into the lower bainite and the martensite structures. After the second tempering process, the structure of the forgings 4 is transformed into a mixed structure of lower bainite and tempered martensite. The mechanical properties, such as toughness, elongation, tensile strength, etc., of the lower bainite are exceptionally good. The structure of the tempered martensite, on the other hand, can enhance strength and hardness, so that the forgings 4 have excellent toughness and hardness. Hence, forgings of large dimensions can be suitably produced with good mechanical properties.

While the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is understood that this invention is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements. 

1. A method of producing forgings having a high tensile strength and an elongation, comprising: (a) forming forgings from a steel wire rod; (b) heating the forgings to a temperature range of 830˜900° C.; (c) subjecting the forgings to first tempering at a temperature range of 100˜300° C. after the heating at 830˜900° C.; and (d) subjecting the forgings to second tempering at a temperature range of 300-400° C. after the first tempering.
 2. The method of claim 1, wherein the forgings are quenched in a salt bath having a temperature in the range of 100˜300° C. during the first tempering, and are tempered for 60˜130 minutes.
 3. The method of claim 2, wherein the forgings are quenched in a salt bath having a temperature in the range of 300˜400° C. during the second tempering, and are tempered for 30˜150 minutes.
 4. The method of claim 3, wherein the forgings are heated to 830˜900° C. for 30˜120 minutes.
 5. The method of claim 1, further comprising the step of preheating the forgings at a temperature range of 550˜650° C. for 30˜90 minutes before the forgings are heated to 830˜900° C.
 6. The method of claim 1, wherein the steel wire rod is made of a hardened wire material selected from the group consisting of nickel-chromium-molybdenum steel and chromium-molybdenum steel.
 7. The method of claim 1, wherein the forgings in step (a) are formed by employing the steps of: subjecting the steel wire rod to a first spheroidized annealing; picking and coating the steel wire rod after the first spheroidized annealing; subjecting the steel wire rod to intermediate drawing after the picking and coating of the steel wire rod; subjecting the steel wire rod to a second spheroidized annealing after the intermediate drawing; picking and coating the steel wire rod a second time; subjecting the steel wire rod to skin-pass drawing after the picking and coating of the steel wire rod a second time;.and stamp-forging and roll-threading the steel wire rod, thereby forming the forgings. 